In order to improve the performance of HBT devices, it is desirable to lower the lateral sheet resistance of the HBT base. High performance InP DHBTs and other HBTs require the base transit time (tb) to be scaled appropriately in order to improve the small-signal unity current gain frequency, (fT) of the device. A smaller tb provides a higher fT. At the same time, the lateral base resistance needs to be scaled appropriately to reduce the base resistance (Rb) and improve the small-signal unity power gain frequency (fMAX), which indicates the maximum frequency at which useful power gain can be expected. The prior art approach is to make the base thinner to reduce tb, but a thinner base negatively impacts Rb, because a thinner base increases the base resistance Rb.
Another approach in the prior art is to increase the doping concentration in the base to reduce Rb while thinning the base as much as possible to reduce tb. However, a high doping concentration leads to degraded hole mobility and decreased DC current gain, because of increased Auger recombination in which an electron and hole recombine. In developmental laboratories bases have been thinned to 20 nm and the base doping concentrations have been increased to 1*1020 cm−3; however, these are extreme limits and are not considered realistic for a commercial InP DHBT process technology.
Another approach in the prior art has been to investigate DHBTs with GaAsSb bases due to the potentially high hole concentrations (>1*1020 cm−3) and the minimal conduction band offset to InP achievable with this material. However, the high hole concentration comes with reduced hole mobility relative to p-type InGaAs alloys. The electron mobility in p-type GaAsSb is also inferior to p-type InGaAs, which limits the tb and DC current gain for a GaAsSb base. The one true advantage of GaAsSb is the minimal conduction band offset of GaAsSb to InP.
Most of the industry has adopted DHBTs with an InGaAs alloy base. High doping concentrations (8*1019 cm−3) with carbon doping have been achieved without a severe penalty to the hole mobility and the electron transport in p-type InGaAs, especially when base layers with a field gradient are used. However, the 150-200 meV conduction band offset to InP requires that the base-collector junction be graded. The presence of this grade complicates the growth of the HBT material layers and can potentially limit the HBT performance. By optimizing the transition from the InGaAs alloy base into the InP wide bandgap collector, fT and fMAX values in excess of 400 GHz and 400 GHz, respectively, at low VCB can be achieved.
What is needed are HBTs and methods for fabricating HBTs with bases with reduced Rb without increasing the base thickness. Also needed are HBTs and methods for fabricating HBTs with bases with reduced tb to provide high fT and fMAX devices. The embodiments of the present disclosure answer these and other needs.